powerpc/44x: Fix UART2/3 interrupt assignment in PPC460EX/GT dts files

UART2 and UART3 on 460EX/GT have incorrect interrupt mappings right now.
UART2 should be 28 (0x1c) and UART3 29 (0x1d). This patch fixes this and
switches to using decimal number instead of hex, since the AppliedMicro
(AMCC) users manuals describe their inerrupt numbers in decimal.

Thanks to Fabien Proriol for pointing this out.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Fabien Proriol <Fabien.Proriol@jdsu.com>
Cc: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>